Transmitted by Bukhari (no.580) and Muslim (no.607) from the hadith of Malik.

[UNDERSTANDING FROM THE HADITH]

1) Abdullah Ibn Umar (RadhiAllahu Anhu) used to say, “If your rakah has been missed, so has the sajdah.” (Muwatta Imam Malik (1/10) hadith (no.15) and its chain is authentic)

2) There is a difference of opinion in regards to the ruku (bowing) if it is a part of the rakah or not, however that which is closest and correct to us in accordance to the authentic ahadith and understanding of the Salaf us-Salihin is that the person who catches the ruku does not catch the rakah because a very important pillar of the Salah has been omitted, meaning Surah Fatiha, and without it the Salah is not accepted. Abu Hurayrah (RadhiAllahu Anhu) said, “Your rakah is not complete until you catch (join the salah with) the Imam in Qiyam (standing position) before the ruku (bowing). (Juz al-Qirah of Bukhari: (no.132), and its chain is [Hasan] good). Abu Sa’id al-Khudhri (RadhiAllaahu Anhu) said, “None of you should do ruku (bowing) without reciting Surah Fatiha. (Juz al-Qirah of Bukhari: (no.133), and its chain is authentic)

3) The Messenger of Allah (Sallalahu Alayhi Was-Sallam) said, “Whoever catches (joins with the Imam) one rakah (of the Jum’ah Salah) on the day of Jum’ah then he has caught the Salah and he should add another rakah to it. (Sunan ad-Daraqutni (2/13) hadith (no.1592), and its chain is [Hasan] good)

From this hadith it is known (and understood), whoever does not catch even one rakah (of Jum’ah Salah) on the day of Jum’ah then he will have to pray the complete 4 Rakahs (for Zuhr). As for the narration which has been transmitted in Akhbar Isbahan of Abu Na’im al-Isbahani (2/200) that the person who does not catch any rakah for Jum’ah then he will (also) pray 2 rakahs is [Da’if] (weak) because Muhammad Ibn Nuh Ibn Muhammad ash-Shaybani as-Simsar is majhul ul-Hal (his condition is unknown).